This invention relates to a method for injection moulding.
Most raw materials utilized in an injection moulding machine or ejection moulding machine contain or evolve gas, air or moisture which are heated, evaporated and expanded while the raw materials are heated or melted in a heated cylinder of the moulding machine and such gas or vapor disperses and remains in the moulded articles as gas bubbles. To overcome this dificulty a so-called vent moulding machine is often used. In the ejection moulding machine a screw is continuously rotated in a heated cylinder so as to continuously eject the heated resin therein, whereas in the injection moulding machine, the rotation of the screw is intermittent so that while the screw is being stopped the resin stays in the heated cylinder and the volatile components and other gases expand. Since the pressure of the portion near the vent opening is reduced the resin will overflow through the vent opening thus causing vent up and closing of the vent opening. Accordingly, the vent effect is nullified. For this reason, in the injection moulding machine, while the screw is being stopped, the pressure of the portion near the vent opening is increased to or above the atmospheric pressure for preventing vent up.
Such a method is well known as disclosed in the specification of Japanese Pat. Publication No. 666 of 1971. This method will be described in the following with reference to FIGS. 1 and 2 of the accompanying drawings.
Thus, in an injection moulding machine, a valve 3 including a passage 2 is provided at a vent opening 1. The valve 3 is rotated for communicating the passage 2 with an inlet opening 5 or an exhaust opening 4. During rotation of screw 6 the valve 3 is maintained in the position shown in FIG. 1 so as to communicate the passage 2 with exhaust opening 4 which is connected to a vacuum pump (not shown) thus maintaining a reduced pressure in the vent opening 1. After completion of metering, rotation of screw 6 is stopped and the valve 3 is rotated to the position shown in FIG. 2 thus introducing to the vent opening 1 air under atmospheric pressure or pressurized inert gas through inlet opening 5 and passage 2, thus preventing vent up.
It should be understood that the rotation of valve 3 is effected by electric signals generated in synchronism with the start and stop of screw 6.
Such prior art method, however, has the following disadvantages.
1. When increasing the pressure at the vent opening to the atmospheric pressure or above the atmospheric pressure which has been maintained under a reduced pressure, it takes a certain time before the pressure at the vent opening increases unless inlet opening of large diameter is used. This means that the reduced pressure condition persists a relatively long period after stopping the screw 6, thus causing vent up. Readily oxidizable material such as nylon will be colored or degraded by contact with air. Where the diameter of the bent opening is increased, it is necessary to use a large and expensive valve.
2. Where pressure is applied to the vent opening 1 by air or inert gas, although it is possible to prevent bent up, oxydation of the material is accelerated when pressurized air is used. Where pressurized inert gas is used, while oxidation of the raw material and vent up can be efficiently precluded, inert gas is generally more expensive than air and in addition supplement and maintainance of such gas must be made with care. Accordingly, use of inert gas is not advantageous for small factories.
The time required for the gas pressure to increase to or above the atmospheric pressure can be decreased by switching the valve to the atmosphere side before the screw is stopped, but this decreases the interval in which the reduced pressure is maintained at the vent opening thus decreasing the effect thereof.